Chicago HVAC, explained

How a furnace and ductwork heat a house

How it works. A forced-air system is one closed loop of air. Gas burns inside the furnace's sealed heat exchanger; the blower pulls house air in through the return ducts and the filter, pushes it across the hot exchanger, and sends it out through the supply trunk and branch ducts to the registers in each room. Cooler air falls back to the return grilles and the loop repeats. Combustion exhaust never joins the loop: it leaves through the flue, up a lined chimney on older units, or out a white PVC sidewall pipe on high-efficiency models that reclaim so much heat their exhaust is barely warm. Central air conditioning is the same loop run backward thermally: a cold coil above the furnace absorbs heat from the passing air and pumps it outside. Radiator buildings do the same job with water instead of air: a boiler heats water, pipes are the ducts, radiators are the registers, and there is no filter because there is no moving air.

Code anchors: furnace and duct work in the city runs under the Chicago Construction Codes (mechanical provisions, Title 18-28) with permits; CO alarms within 15 feet of sleeping rooms per Illinois law. Boilers and radiators get their own affection in the old-home guide.

Forced air vs radiators: the two ways Chicago heats

How it works. Every Chicago home heats with one of two loops. A forced-air system is a loop of air: the furnace warms it, the blower and ducts deliver it, the returns bring it back. A boiler system is a loop of water: the boiler warms it, pipes deliver it to radiators or baseboards in each room, the metal quietly radiates that heat into the room, and the cooled water heads back to the boiler to go around again. No ducts, no blower, no filter, because nothing but water ever moves. That is why vintage buildings with radiators have no ductwork anywhere: the pipes are the ducts and the radiators are the registers.

The trade is comfort versus flexibility. Radiator heat is even, silent, and gentle, it does not blow dust around or dry the air out, and a cast iron radiator is close to immortal. Forced air responds faster, filters and humidifies the air as a side effect, and, most importantly, gives central air conditioning a free ride: the cold coil borrows the same ducts in summer. A radiator building that wants central cooling has to add it separately, which is why you see window units, ductless mini-splits, or slim high-velocity systems in vintage Chicago housing.

Steam vs hot water: two different boilers. All radiator systems are not the same, and the difference matters at inspection. A hot water (hydronic) boiler keeps the loop completely full of water, heats it to roughly 160 to 180 degrees, and uses a small electric circulator pump to push it around, with an expansion tank absorbing the water's growth as it warms. A steam boiler boils: the steam rises through the pipes under its own pressure, no pump at all, fills each radiator, hands over its heat as it condenses back into water, and drains home by gravity. You can tell them apart at the boiler: steam shows a glass water-level tube (the sight glass), a pressure gauge reading in single-digit psi, and a low-water cutoff; hot water shows a temperature gauge, a circulator pump, and that expansion tank. The radiators tell you too: one-pipe steam radiators wear a little side-mounted air vent that hisses softly as the system fills, while hot water radiators have a small bleed valve at the top that gets "burped" with a key each fall to release trapped air.

Boiler notes for buyers: hot water boilers commonly serve 15 to 30 years and steam boilers, especially cast iron, often longer; both want annual service, and on steam the low-water cutoff is the safety that matters. Banging pipes on steam (water hammer) is usually a slope or vent fix, not a catastrophe. The radiators themselves almost never need replacing. More vintage-heating lore in the old-home guide.

How central air cools: meet the condenser

How it works. Air conditioning does not create cold; it moves heat from inside the house to outside it, using a refrigerant that boils at very low temperatures. Inside, at the coil above your furnace, liquid refrigerant passes through a tiny metering valve and its pressure drops, which makes it boil, and boiling absorbs heat, so it soaks heat out of the household air the blower pushes across the coil (moisture in the air condenses on the cold coil too, which is where summer humidity goes, down the condensate drain). The now-warm refrigerant vapor travels out to the condenser, the humming box beside the house. There the compressor squeezes the vapor, concentrating its heat until it is hotter than a summer afternoon, and the big fan blows outdoor air across the condenser coil so the heat leaves into the yard while the refrigerant condenses back into a liquid. The liquid heads back inside and the loop repeats, hundreds of times an hour, all summer.

Condensers in our climate typically serve 12 to 18 years. Code wants a shutoff (the small "disconnect" box) within sight of the unit, and the newer refrigerant transitions mean a very old unit that fails is usually replaced rather than repaired, a budgeting fact worth knowing before you waive anything.

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Cooling a radiator building: the SpacePak answer

Here is the vintage-Chicago dilemma in one sentence: half the housing stock heats with hot-water radiators, which means there is no ductwork anywhere in the building, and conventional central air needs big sheet-metal ducts that would mean tearing open plaster, dropping soffits through rooms, and giving up closets. For decades the answer was window units and resignation. Then came small-duct high-velocity systems, which everyone calls SpacePak the way every tissue is a Kleenex (Unico is the other big name).

How it works: a compact air handler tucks into an attic, a closet, or a basement ceiling, and instead of bulky ducts it pushes air fast through flexible, sound-insulated supply tubes about two inches across. Tubes that small snake through stud bays and joist spaces like thick cable, which is the entire trick: the system threads through a finished vintage building without opening the walls. In the rooms, the outlets are small round ports in a ceiling or floor, closer to a doorbell than a register. The fast-moving air stirs the whole room as it enters, so temperatures stay even and the system wrings out humidity well, which Chicago Julys appreciate.

Why people choose it: it is the whole-home cooling answer for greystones, two-flats, bungalows, and vintage condos with boiler heat, and it preserves exactly what makes those buildings worth owning, the plaster, the trim, the ceilings. The trade-offs: it costs more than conventional ducted air would in a house that already had ducts, the outlets make a soft rush of air that a lazy design turns into a whistle (placement and run lengths are real engineering, so use an installer who lives in these systems), and you still need a spot for the air handler plus a condenser outside. The main alternative is ductless mini-splits, which win on price-per-zone and need no tubing at all, but put a visible cassette on the wall of every room they serve; SpacePak wins on invisibility and on cooling the whole home from one system.